Various methods of performing the nitriding process are known to those skilled in the art. In some cases, the process is performed by providing raw ammonia to the nitriding furnace and sometimes the process is performed by providing the furnace with raw ammonia combined with a carrier gas mixture of nitrogen and hydrogen formed by the dissociation of ammonia in an ammonia dissociator. It is desirable to control the nitriding potential within the furnace. This can be done in a number of ways. For example, it is known that the oxygen partial pressure in the nitriding atmosphere can be measured and used to control the nitride potential provided that the supply of ammonia contains small amounts of impurities in the form of oxygen or oxygen containing compounds such as water. One arrangement is described by S. Bohmer, et al., in Oxygen Probes for Controlling Nitriding and Nitrocarburizing Atmosphere, published in Surface Engineering, v. 10, #2, 1994, pp.129-135. In the Bohmer arrangement, a special probe, referred to as an equilibrium probe (E probe) produced by Process Electronic Company, uses a heated catalyst within the probe to dissociate any residual ammonia of the furnace atmosphere before coming in contact with the furnace atmosphere sensing element of the probe. Problems with the Bohmer suggested sensing system arise from the assumptions that all of the ammonia is dissociated by the heated catalyst prior to contact with the furnace atmosphere sensor of the E probe and that the temperature difference between the furnace atmosphere sensor and the reference sensor are insignificant. In actual practice, both assumptions can be faulty and give rise to errors which are detrimental to accurate process control and the quality of nitrided articles.
More recently, U.K. patent application GE 2,184,549A by Dr. Hans-Heinrich Moebius et al. uses an arrangement of four sensors to control the atmosphere of a nitriding furnace. Separate sensors measure the treatment gas oxygen partial pressure in the furnace atmosphere and a second sensor, in conjunction with a heated catalyst path, supplies a separate, remote, reference gas measurement. This arrangement has the disadvantages of requiring multiple probes, separate temperature measurements, and heated catalytic internal passageways.
Another arrangement is discussed in unexamined patent application DE 42 29 803 A1, by R. Hoffman. The described method requires that a portion of the waste gas stream is shunted out of the furnace and delivered to a separate dissociation unit outside of the furnace space. This arrangement has the disadvantages of requiring an additional dissociation chamber strictly to fully dissociate the furnace sampling gas.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a method for controlling the atmosphere of a nitriding furnace using an oxygen probe which does not require the use of internal catalysts and heating elements to provide a reference gas for the probe. It is also desirable to have such a method which requires only a single probe to control a single furnace. Furthermore, it is desirable to have such a method that does not require additional dissociators beyond those already present to produce a dissociated ammonia carrier gas, heating elements and electrical inputs, thereby enabling the method to be readily used in otherwise conventional nitriding systems. The present invention overcomes the above noted problems by the novel method of using a single conventional oxygen probe supplied with a reference gas taken from the already present dissociated ammonia carrier gas. By using a conventional oxygen probe, the complex dissociator internal to the prior art probes and associated temperature problems are eliminated. Because the dissociated ammonia reference gas is produced in a commercial dissociator designed specifically to produce complete dissociation, the problems with incompletely dissociated reference gas are eliminated.